As well known, flexography is relief printing in which a flexible printing plate is used. The flexography has been reappraised and attracting attention because of its higher printing accuracy, better economic efficiency, versatility, and environmental friendliness.
In Japan, relatively common printing systems are the offset printing and the gravure printing. Other printing systems that are less common than them may include the silk-screen printing. The relief printing such as the flexography is relatively not so common in Japan, although flexography has some advantages such as the following:                (1) The plate used in the flexography has a relief and the plate is flexible which allows printing on objects that are not flat and smooth like the corrugated cartons. In addition, the flexography is faster and cheaper than other printing systems.        (2) The flexography allows printing on smaller objects, such as labels, sharply and cheaply.        (3) Compared with the offset printing, the flexography produces a clearer print because of the possibility of use of a highly concentrated ink.        (4) Compared with the gravure printing, the flexography produces sharper characters and images.        (5) As an aqueous ink and a completely solvent-free ink such as an UV ink can be used, the flexography is environment friendly.        (6) The flexography takes lesser time to attain a stable color condition, and therefore, a futility of print media, such as paper, can be cut down which makes the flexography more economical.        (7) The flexography needs lesser maintenance cost or modification cost because only a part of the plate can be replaced without the need to replace the whole plate.        (8) The flexography can be used in many varieties of small lot production.        
Such a variety of advantages are well known since the relief printing is a conventional old printing technique, and so has been the flexography as a sort of the relief printing. Nevertheless, the flexography was not so common among all of the printing industry.
However, recently, there has been an improvement in the quality of printing ink and printing materials, and a rise of concern about environmental problems so that the relief printing such as flexography is being re-evaluated. Therefore, research and development of the flexography has been intensifying at present.
The state of art as to the printing original plate is as follows:
The flexographic printing plate has been made of a rubber layer as a constituent material, on which a negative image of characters and pictures to be printed are carved. However, recently, a photosensitive resin is being used to make the flexographic printing plate (see, for example, Japanese Patent Publication No. 2916408 and Japanese Patent Application Laid-open No. 2003-35954). In general, this photosensitive resin includes an elastomeric binder, at least one monomer resin, and a photoinitiator. The flexographic printing original plate made with the photosensitive resin is a material in a form of a plate including a substrate and at least a photosensitive resin layer provided thereon.
Production of the flexographic printing plate by using such a printing original plate may be carried out in accordance with the following procedure. A film (mask) that has a negative pattern corresponding to images to be printed, such as characters or pictures, is placed on the photosensitive resin layer of the printing original plate. The photosensitive resin layer is then irradiated with an actinic radiation through this mask. The part of the layer irradiated with the actinic radiation is cured by photopolymerization. Subsequent washout of the uncured part with a developer liquid results in development of a relief pattern corresponding to the images, to give a flexographic printing plate. Upon performing flexography, ink is applied to the top surface of the relief pattern and the flexographic printing plate is pressed on a printing media such as paper, to obtain a print product.
It has been pointed out that the flexographic printing plate made of the photosensitive resin has following problems that are desired to be solved:                (i) When the negative pattern on the mask has to be corrected, whole of the mask has to be re-made since it is impossible to partially correct the mask. Therefore, the correction requires a large number of steps.        (ii) As the negative mask is made of a negative film, the shape of the mask tends to be fluctuated due to changes in the temperature and the humidity. Therefore, even if the same mask is used, accuracy of the printing may change depending on the time and environment for performing pattern formation steps including exposure and development of the photosensitive resin layer.        (iii) Optical obstacles such as dust enter easily between the negative mask and the photosensitive resin layer during the pattern forming process. Such optical obstacles between the mask and the photosensitive resin layer may cause inaccuracy of the pattern image after exposure and development, which may lead to deterioration of the printing quality of the printing plate.        
As a countermeasure to these problems, there have hitherto been developed several kinds of printing plates each having a new construction (see, for example, Japanese Patent Publication No. 2916408, Japanese Patent Application Laid-open No. 2003-35954, Japanese Patent Application Laid-open No. 2003-35955, Japanese Patent Application Laid-open No. H11-153865, Japanese Patent Application Laid-open No. H9-166875, Japanese Patent Application Laid-open No. 2001-324815, and Japanese Patent Publication No. 2773981). A common feature of these printing original plates is that an infrared-sensitive layer is at least formed on the photosensitive resin layer. The infrared-sensitive layer may be used as a negative mask or a positive mask. This infrared sensitive layer is opaque to actinic radiations that polymerize the photosensitive resin in the photosensitive resin layer, but has a photosensitivity to infrared radiations. The infrared-sensitivity is the ability to be vaporized and/or decomposed, i.e. ablated, by the exposure to the infrared laser radiations. Therefore, the infrared-sensitive layer is also referred to as an infrared ablation layer.
Lamination of such an infrared ablation layer enables direct recording of the image information to be printed with an infrared laser on the relief printing original plate such as the flexographic printing original plate, and thus obviates the need of the conventional negative or positive mask film. The print image information corresponding to the print pattern can be formed, saved, corrected, and output as a digital information using a computer. The infrared ablation layer thus drastically reduces the cost for image information processing which is necessary when the conventional mask film is used for producing the relief printing plate.
Various kinds of compositions are known for the infrared ablation layer, however, the basic composition is similar. For instance, Japanese Patent Publication No. 2916408 discloses a printing original plate having an infrared ablation layer made of (1) at least one infrared absorbing material, (2) an opaque material to non-infrared radiations (ultraviolet radiations), wherein the infrared absorbing material and the opaque material may be the same or different, and (3) at least one binder that is substantially incompatible with low molecular materials in the photosensitive resin layer.
Any material that prevents the transmission of non-infrared actinic radiations to the photosensitive resin layer may be used as the opaque material. Examples of suitable opaque materials include dyes that absorb ultraviolet or visible light, dark inorganic pigments and combination thereof. Preferred opaque materials are carbon black and graphite.
The infrared absorbing material has a strong absorption in the region from 750 to 20,000 nm. Examples of suitable infrared absorbing materials include dark inorganic pigments such as carbon black, graphite, copper chromite, chromium oxides, and chromium-cobalt aluminate; and dyes such as poly(substituted)phthalocyanine compounds, cyanine dyes, squarylium dyes, chalcogenopyryloarylidene dyes.
With the conventional relief printing original plate, the mask layer that is opaque to the ultraviolet actinic radiation is selectively ablated (burned out) for forming an area of the mask through which the ultraviolet actinic radiation can pass. In the step of ablation of the mask layer for obtaining the mask image layer, it is thus necessary to provide a vacuuming and exhausting means for taking ablation debris out of the reaction system in order to prevent the ablation debris from remaining on or attaching to the surface of the photosensitive resin.
If even a thin layer of the mask material remains in the area of the mask image layer through which the actinic radiation has to pass, the amount of the actinic radiation that pass through the mask will be decreased. Therefore, the ablation of the mask layer must be performed until the photosensitive resin layer is revealed. Such an ablation process results in roughening of the revealed surface of the photosensitive resin layer by the infrared laser. Since such roughening on revealed portion remains in the subsequent development step to be the surface on which ink for printing will be put, it may cause damage on the quality of the printing.